more complete handling of corner-case roundings + add proper tests

afvincent · afvincent · commit c98ba91dbe01 · 2017-08-25T09:39:46.000-07:00
diff --git a/lib/matplotlib/tests/test_ticker.py b/lib/matplotlib/tests/test_ticker.py
@@ -564,11 +564,13 @@ class TestEngFormatter(object):
         (0.1, ('100 m', '100 m', '100.00 m')),
         (1, ('1', '1', '1.00')),
         (1.23456789, ('1.23457', '1', '1.23')),
-        (999.9, ('999.9', '1 k', '999.90')),
+        (999.9, ('999.9', '1 k', '999.90')),  # places=0: corner-case rounding
+        (999.9999, ('1 k', '1 k', '1.00 k')),  # corner-case roudning for all
         (1000, ('1 k', '1 k', '1.00 k')),
         (1001, ('1.001 k', '1 k', '1.00 k')),
         (100001, ('100.001 k', '100 k', '100.00 k')),
-        (987654.321, ('987.654 k', '988 k', '987.65 k'))
+        (987654.321, ('987.654 k', '988 k', '987.65 k')),
+        (1.23e27, ('1230 Y', '1230 Y', '1230.00 Y'))  # OoR value (> 1000 Y)
     ]
 
     @pytest.mark.parametrize('input, expected', raw_format_data)
diff --git a/lib/matplotlib/ticker.py b/lib/matplotlib/ticker.py
@@ -1265,6 +1265,7 @@ def format_eng(self, num):
         """
         dnum = float(num)
         sign = 1
+        fmt = "g" if self.places is None else ".{:d}f".format(self.places)
 
         if dnum < 0:
             sign = -1
@@ -1283,17 +1284,18 @@ def format_eng(self, num):
 
         mant = sign * dnum / (10.0 ** pow10)
         # Taking care of the cases like 999.9..., which
-        # may be rounded to 1000 instead of 1 k.
-        if (self.places is not None and
-                round(mant, self.places) >= 1000):
+        # may be rounded to 1000 instead of 1 k.  Beware
+        # of the corner case of values that are beyond
+        # the range of SI prefixes (i.e. > 'Y').
+        _fmant = float("{mant:{fmt}}".format(mant=mant, fmt=fmt))
+        if _fmant >= 1000 and pow10 != max(self.ENG_PREFIXES):
             mant /= 1000
             pow10 += 3
 
         prefix = self.ENG_PREFIXES[int(pow10)]
 
         formatted = "{mant:{fmt}}{sep}{prefix}".format(
-            mant=mant, sep=self.sep, prefix=prefix,
-            fmt="g" if self.places is None else ".{:d}f".format(self.places))
+            mant=mant, sep=self.sep, prefix=prefix, fmt=fmt)
 
         return formatted
 
